ELECTROLYTIC CHLORINATION AND pH CONTROL OF WATER

ABSTRACT

A swimming pool water sterilizer and the method involved in the operation are disclosed employing a two electrode compartment electrolytic cell with said compartments being separated by a cation selective membrane. A concentrated alkali metal chloride salt solution is fed to the anode compartment wherein on application of a decomposition voltage there is formed therein essentially chlorine gas and hypochlorous acid. The migration of water from the anode compartment to the cathode compartment as a result of the solvation of the alkali metal ions (Na ) which passes to the cathode compartment through the cation membrane allows the formation of a caustic solution and hydrogen gas within the cathode compartment. The effluents resulting from both compartments are fed into the recirculated pool water for sterilization. The pH of said water can be controlled by discharging to waste any excess basic catholyte at predetermined intervals at pre-set periods of time.

[ 1 June 13, 1972 ELECTROLYTIC CHLORINATION AND PH CONTROL OF WATERThomas A. Kirkham, Lexington; John W. Arnold, Wilmington; Anthony J.Giuffrida, North Andover, all of Mass.

Ionics Incorporated, Watertown, Mass.

July 30, 1970 [72] Inventors:

Field of Search ..204/149, 98;2l0/192, 169

[56] References Cited UNITED STATES PATENTS l l/1965 Osborne ..204/98 X6/1968 Cooper ..204/98 X 2/1971 Richards et a1. ..204/272 ACIDIC ANOLYTEAND CHLORINE GAS Primary Examiner-John l-l. Mack Assistant ExaminerA. C.Prescott Attorney-Norman E. Saliba and Aaron Tushin 57 ABSTRACT Aswimming pool water sterilizer and the method involved in the operationare disclosed employing a two electrode compartment electrolytic cellwith said compartments being separated by a cation selective membrane. Aconcentrated alkali metal chloride salt solution is fed to the anodecompartment wherein on application of a decomposition voltage there isformed therein essentially chlorine gas and hypochlorous acid. Themigration of water from the anode compartment to the cathode compartmentas a result of the solvation of the alkali metal ions (Na*) which passesto the cathode compartment through the cation membrane allows theformation of a caustic solution and hydrogen gas within the cathodecompartment. The effluents resulting from both compartments are fed intothe recirculated pool water for sterilization. The pH of said water canbe controlled by discharging to waste any excess basic catholyte atpredetermined intervals at pre-set periods of time.

6 Claims, 2 Drawing Figures CAUSTIC CATHOLYTE AND HYDROGEN GAS (NciCll(NciOHl PATENTEflJun 1 a ma ACIDlC ANOLYTE AND CHLORINE GAS CAUSTICCATHOLYTE AND HYDROGEN GAS FEED SOLUTION WASTE SOLUTION (NaCl) (NQOH) 4AC. T0 PUMPS e9 9 g 3 SOLENOID. ETC.

y I (x) i 5 W 9 i g P i i A-32 11 1o POWER SUPPLY METERS GOCYCLES TIMER,ETC.

Plg. Z

INVENTORS THOMAS A. KIRKHAM JOHN W. ARNOLD ANTHONEY JGIUFFRIDA ATTORNEYELECTROLYTIC CHLORINATION AND PH CONTROL OF WATER This invention relatesto a process and system for continu ously sterilizing water and moreparticularly to the electrolytic production of chlorine for use in thesterilization of swimming pool waters wherein the desired chlorineresidual and desired pH level of the water is continuously maintained.The invention is also applicable for disenfecting food processingplants, laundry water, municipal water supplies etc.

Chlorine is a well known disenfectant commonly employed for use inswimming pools and other water supplies for sterilization and for thecontrol of algae. The most common method of treating small swimming poolwaters involves the frequent addition of unstable alkaline reactingcompounds such as sodium or calcium hypochlorite which on decompositionreleases nascent oxygen which is generally considered (See U.S. Pat. No.3,152,073) the effective sterilizing agent as follows:

NaClO NaCl Reaction (I) The addition of the hypochlorite which isusually contaminated with hydroxides results in the undesirable sideeffect of increasing the alkaline content of the water which thennecessitates the addition of an acid or acid salt to control the pHlevel where desired. In this method there is therefor required thenecessity for the direct handling of hazardous liquids or solidchemicals with little if any control of the pH of the water.Additionally, the use of calcium hypochlorite will often cause thedevelopment of turbidity in the water due to the formation of calciumcarbonate.

The direct addition of gaseous chlorine especially in the largercommercial pools is also employed for sterilization but its use must besurrounded by safe guards since the gas is under great pressures inheavy steel tanks and is highly corrosive and toxic. The chemicalreaction in the use of gaseous chlorine is believed to be as follows:

C1 H O HCl HCIO Reaction HCIO HCl (0) Reaction (III) Again the nacentoxygen released from the hypochlorous acid is probably the effectivesterilizing agent. The direct addition of chlorine gas tends to lowerthe pH of the water since acid (I-lCl) is obtained when the gas reactswith water to produce nacent oxygen. Sodium carbonate or other alkalinechemicals must be added to raise the pH within the safe range of betweenabout 7.0 to 7.8 or preferably between 7.2 to 7.6.

The prior art also resorted to the use of an electrolytic cell forproducing the required dosages of residual chlorine in the waterobtained from the decomposition of chloride salts added directly to thepool water. This method however, was not successful in controlling thepH level but resulted in an increase of pH due to excessive causticbuild up in the pool. These electrolytic cells required frequentservicing due to damage often resulting from excessive current densitiesand excessive cell temperatures.

The above description shows the inherent disadvantages of all thepresent methods and apparatus used in swimming pool treatment. Thesedisadvantages are overcome by the present novel process and apparatuswhich functions to maintain the pH level and the chlorine concentrationwithin a predetermined range.

Accordingly, it is a primary object of this invention to provide aelectrolytic cell system which decomposes or breaks down an alkali metalhalide preferably the chloride salt such as sodium chloride (commonsalt) to its basic elements and allows these elements to recombine toform a purifying combination of chemical compounds which are thenemployed to chlorinate swimming pool waters.

It is a further object of this invention to avoid the handling ofhazardous chemicals by the use of an inexpensive alkali metal halidesalt as the only added chemical in the operation of the electrolyticcell.

It is a further object of this invention to provide electrolyticchlorination of a body of water such as a swimming pool wherein the pHof said water is controlled to a desired level.

A further object is to provide an electrolytic cell in the system thatseparately generates acidic chlorine solution and basic causticseparately and then combines them in the proper proportion to maintainthe desired pH of a body of water.

It is a further object of this invention to provide means forcontrolling the injection of chlorine and caustic into a body of waterwhereby the chlorine residual and the pH of the water is maintained atthe desired levels without requiring the addition of adjustingchemicals.

A further object is to provide separate control of the rate and quantityof available acidic chlorine solution and basic caustic generated in theelectrolytic cell of the system.

It is still another object of this invention to provide an inexpensiveswimming pool water sterilizer and conditioner which is designed forefficient and substantially maintenance-free operation.

A better understanding of the present invention may be obtained by theappended drawings in which:

FIG. 1 is a diagramatic, vertical cross-section representation of oneembodiment of electrolytic cell; and

FIG. 2 is a schematic illustration of a preferred swimming pool waterpurification system which incorporates the electrolytic cell of FIG. 1.

Referring to the drawings in detail the electrolytic cell (X) of FIG. 1,(which is used in the system of FIG. 2 and so designated with likenumerals) is shown to be divided into a cathode and an anode compartment1 and 2. Preferably a porous diaphragm (D) formed of a substantiallyacid and chlorine resistant material such as Teflon is secured adjacentto the substantially non-porous ion-exchange membrane (M) to protect thelatter from deleterious products of the anode reactions. A chemicallyresistant plastic screen (S) may also be placed in direct contact withthe cathode side of the membrane to give support thereto againstincreased pressure originating from the anode compartment. It iscontemplated that a further embodiment of the cell would employ amicroporous diaphragm such as polypropylene, asbestos etc. in place ofthe cation membrane although the efficiency of the cell would suffer bysuch a substitution. The introduction for example of saturated brine (NaCl) at feed inlet 11 into anode compartment 2 and the application of adecomposition potential across the electrodes (cathode 3 and anode 4)results in an electrochemical reaction. In such a reaction the sodiumion (Na migrates through the cation selective membrane (M) to formsodium hydroxide by combining with the OH ion which remains in thecathode compartment 1 when hydrogen gas is released by the cathodicreaction.

The cation membrane employed is preferably of the type containingcarboxylic active groups as fully described in U.S. Pat. No. 2,731,408.Cation exchange membranes which depend upon a carboxyl group for theiractivity are prepared by forming insoluble polymers, co-polymers, orheteropolymers, of an unsaturated carboxyl containing compound or theequivalent thereof. If the starting materials are carboxylic acids, thefunctional groups of the acid are obtained in the acid form. If an acidanhydride is used, acid groups are readily formed by reaction of theresin with water. Ester groups can be hydrolyzed or saponified to yieldresins with free carboxylic groups. Salt forms can be converted to thecarboxyl forms by treatment of the resin in the salt form with an acidsolution. A specific type of a carboxylic exchanger is prepared byheteropolymerizing maleic anhydride or fumaric acid with styrenetogether with a cross-linking agent such as divinylbenzene. Otherparticularly useful starting materials for preparing carboxylic resinsare acrylic acid and methacrylic acid. If these materials arepolymerized into an insoluble form, the resulting products are of highcapacity. The aforesaid acids may also be co-polymerized withpolyunsaturated polymerizable compounds such as divinylbenzene,trivinylbenzene, ethylene diacrylate, or dimethylacrylate, diallylmaleate or fumarate to yield insoluble carboxylic resins effective ascation exchangers. Additionally, phenolic sulfonic acid, polystyrenesulfonic acid, or polystyrene sulfonic acid containing activatedhydroxyl groups can be used in the preparation of cation exchangemembranes for use in the present invention.

It is to be noted that during cell operation sufficient water will betransferred with Na through the cation membrane from compartment 2 intocompartment 1 to form the catholyte of sodium hydroxide. This watertransfer which is inherently associated with ion transport results fromtwo causes: (1) since all ions are more or less hydrated, watertransport is inherent in the transfer of ions; and (2) since themembrane contains a fixed charge, there is an endosmotic water transferunder the influence of an imposed DC potential across the membrane. Noother water is supplied to said cathode compartment 1 from any othersource during the operation of the electrolytic cell. The chloride ion(Cl) in the anolyte is substantially excluded from entering the cathodecompartment 1 by the cation selective membrane (M) as shown by the arrowin the drawing. The anodic action forms chlorine gas (Cl which reactswith water to form hypochlorous acid (HClO) in the anode compartment 2as shown by reaction (II). A porous sheet of Teflon, (trademark)protects the membrane from the deleterious effects of the chlorine gas.By cathodic action the generation of the hydrogen gas (H in the cathodecompanment produces a positive pressure resulting in the displacement ofthe caustic catholyte (NaOH) out at the top of the cell through conduit5. Similarly, in anode compartment 2, the evolved chlorine gas alsodisplaces the anolyte of hypochlorous acid and unused brine throughconduit 6. When the anolyte and catholyte mix the dissolved chlorine andfree chlorine gas react with the caustic to form hypochlorites.

Cl: 2 NaOH NaClO NaCl H O Reaction In addition at least a portion of thecaustic catholyte in compartment 1, is removed or drained through wasteconduit 9 at controlled preset time intervals from the said cathodecompartment through a solenoid operated valve 10 which operation will bedescribed in more detail hereinafter. During operation of theelectrolytic cell both electrodes (but especially the cathode because ofits low water intake) will generate heat and the cell becomes quitewarm. In order to function efiiciently and to prevent cell damage,necessary cooling is provided by passing a cooling fluid such as waterthrough a confined area or chamber 12 associated with the back side ofthe cathode 3 through liquid cooling inlet and outlets 7 and 8,respectively. The cooling may also be effected, for example, by passinga coolant through a coil located directly in the cell compartments. Theanode electrode may also be cooled in the same manner as described forthe cathode if so desired.

Correllation of the described electrolytic cell of FIG. 1 into thecomplete system for treating swimming pool water is shown in FIG. 2which will now be described in detail.

Attached to the two upper located outlet conduits 5 and 6 of theelectrolytic cell of FIG. 1 is a mixing area, zone or chamber throughwhich the water from swimming pool 21 recirculates through conduit 29between pool inlet and outlets 23-24 by means of a pump 26, throughfilter 25, through the mixing area 20, and back to the swimming pool 21.

The circulation of the cooling water for the cathode 3 of theelectrolytic cell is most expeditiously effected by use of the poolwater flowing from take-off point 27 through cooling inlet pipe 7 andinto return entry point 28 from cooling outlet pipe 8. Conduits 5 and 6located at the upper sections of the cell are preferably made of tubularplastic materials such as Teflon having, for example an inside diameterof between oneeighth to one-fourth inch but preferably aboutthree-sixteenths inch. This diameter size was found ideal where thecirculating pool water flow rate would average (as in the case of amedium size swimming pool) between 20 and gal/min. It was alsounexpectedly found that the length of the conduit tubes 5 and 6 was alsocritical for proper operation of conveying the gases and fluids from theelectrode compartments into the mixing area 20. It was determined thattheir lengths should be at least about 6 inches and preferably fromabout 9 to 12 inches. The reasons for this criticality is not clear butit is found that when the tube lengths are shorter than about 6 inchesthe gases and cell fluids would often recede back into the electrodecompartments accompanied by the entry and mixing with pool water. Itwould also be apparent that this length dimension will vary with theinternal diameter size of the conduit tubes, the current density, pumppressure, water recirculation rates etc. and obviously variations inthese parameters are to be expected in accordance with the volume of theswimming pool water being treated. Provision is also made during cellshut-down periods to drain at least a portion of the caustic catholytepreferably from the bottom of the cathode compartment 1 via conduit 9,through selenoid valve 10; the opening and closing of this valve beingcontrolled by a timer located in power box 32. This automatic drainingof caustic solution provides the novel means for controlling the pHlevel of the swimming pool water since the injection into the water ofthe entire electrolytic cell effluent products would result inincreasing the pH level of the pool water due to the gradual build-up ofexcess caustic solution therein necessitating acid addition as notedhereinabove. This pH build-up has been proved true with all previoustypes of electrolytic cells used for sterilizing swimming pool water.However, the present invention using periodic draining of the catholytecontent during predetermined shut-down periods results in the pH of theswimming pool being held substantially constant. This present systemwhich permits the controlled splitting of the products of saltelectrolysis and a withdrawal of a percentage of the caustic produced towaste provides a means of accurately controlling and maintaining the pHof the swimming pool water at any desired level. The power box 32contains the power supply, meters and the timer controlling the solenoidvalve 10 for withdrawing at set intervals (such as once each day) atleast part of the caustic contents of the cathode compartment asreferred to above. The power supply (1 10 V, AC input) supplies a fixedrectified DC voltage output to the electrolytic cell as well as ACcurrent for the pumps, timer etc. used in the operation of the system.The saturated brine solution in salt reservoir 31 is passed by pumpingmeans 30 through a filter 34 and a flow metering device 33 at apredetermined rate into the anode compartment 2 of the electrolyticcell. AC power for pump operation is supplied from the power supply andhence, ensures delivery of brine to the anode compartment when a DCpotential is applied to the cell. The solenoid valve 10 on the causticsolution outlet located ideally at the lower section of the cathodecompartment permits extraction of pre-set volumes of caustic atpredetermined time intervals as controlled by the timer. As detailedabove, the extraction of a percentage of the caustic produced at thecathode permits absolute control of the pH of the pool water into whichthe electrolytic cell products are injected. The frequency of extractionof caustic via the solenoid valve 10 which is preferably located at thebottom of the cell can be readily determined and is most expeditiouslyeffected during night time shut-downs. The electrolytic cell operatesonly when the pool water recirculation pump is operating which meansthat it will be started and stopped at least once a day. The followingexample will serve to illustrate the actual operation of the presentinvention but is not meant to limit the invention thereto.

A sterilization system was constructed essentially in accordance withthat illustrated in FIG. 2. The electrolytic cell constructed of twoplastic halves had outside dimensions of approximately 7 7 4 inches fortreating a swimming pool containing from about 17,000 to 20,000 gallonscapacity. The electrodes located in the plastic end plates were circularin cross section, 4 inches in diameter and spaced about 2 Va inchesapart from each other. The anode was constructed of platinized titaniumas described in US. Pat. No. 3,117,023 and the cathode of stainlesssteel. The homogeneous cation selective membrane was of the preferredcarboxylic type such as disclosed in U.S. Pat. No. 2,731,408. Suchmembranes as previously stated comprise copolymers of polyvinyl aromaticcompounds and olefinic carboxyl monomers although other type cationmembranes well known in the art may still be employed. A protectivesheet of tightly woven but porous Teflon cloth was placed adjacent tosaid membrane on the anode side and a plastic screen on the cathodeside. The feed solution to the anode compartment was a saturatedsolution of common salt amounting to about 0.65 gallons per day or 1.7lbs salt per day of which about 50 percent of this salt was consumed inthe electrochemical reaction with current efficiencies in the 75 percentrange. The direct electric current supplied to the cell averaged about 8amperes with an average cell voltage maintained at about 6 volts. Theswimming pool water recirculation pipe was 1 4 inches in diameter andthe mixing area or chamber portion about 2 inches in diameter. Theoutlet conduits or tubes from the top portion of the anode and cathodecompartments were made of Teflon (trademark) having an inside diameterof three-sixteenths inch, and a length of about 10 inches and extendedfrom the top of the electrolytic cell to the recirculating pool waterconduit (mixing zone or chamber) with said water being recirculated atabout -25 gals per minute.

The system operated daily from about 3 hours in the winter time to fromabout 12 to almost 24 hours in the summer time. The cell supplied anaverage of about 0.4 lbs of chlorine per day with the pH of the poolwater varying from 7.2 to 7.8, The control of this pH was efiected bythe timer which operated the solenoid valve to allow discharge ofcatholyte from the cathode chamber for about 10-30 seconds each dayconviently during a night shut-down time depending on the measuredacidity of the pool water. This system which operated for one year gaveexcellent results and produced clean sterile water at comfortable wateracidities and with complete lack of algae infestation.

While particular embodiments of the present invention have beendescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and therefore, the aim in the appended claims is tocover all such changes and modifications as come within the true spiritand scope of this invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A system for chlorinating and controlling the pH of swimming poolwater; the pool having water inlet and outlet means connected to a waterrecirculation conduit having pumping means in association therewith foreffecting flow of pool water therethrough comprising:

a. an electrolytic cell having a cation permselective membraneseparating the cell into a cathode and anode compartment containing,respectively, a cathode and anode electrode therein, with at least oneof said electrode com partments being associated with cooling means forremoving heat therefrom;

b. a salt feed conduit connecting a salt solution source means to saidanode compartment and pumping means for introducing said salt solutioninto contact with said anode to form the anolyte solution;

. said cathode compartment having a discharge conduit in associationwith valve means, said valve means connected to timer means forcontrolling the opening and closing of said discharge conduit at pre-setdesired time intervals for withdrawing caustic catholyte therefrom;

. a mixing zone or area section spaced above the top level of theelectrolytic cell and interposed in the pool water recirculatingconduit;

e. gas-liquid conduit means originating from the upper portion of eachof said anode and cathode compartments and in communication with saidmixing zone or area section for discharging gases and catholyte andanolyte liquids into said zone, power supply means for passmg a directcurrent through the electrodes of said cell, and current means foroperating the said pumping and timer means.

2. The system of claim 1 wherein the mixing zone section issubstantially larger in cross section than the pool recirculatingconduit,

3. The system of claim 1 wherein at least the side of said cationselective membrane facing the anode is protected from attack by theproducts of the anodic reaction by a porous, substantially inertdiaphragm in contact therewith.

4. The system of claim 1 wherein said cooling means comprise a confinedspace located in the wall of said cell and in association with the backside of an electrode, said space being in communication with means forcirculating a cooling fluid therethrough.

5. The system of claim 4 wherein the cooling fluid is water obtainedthrough conduit means communicating with the said pool waterrecirculation conduit.

6. The system of claim 1 wherein the said gas-liquid conduit tubesmeasure internally about three-sixteenth inch in diameter and between 9to 12 inches in length.

2. The system of claim 1 wherein the mixing zone section issubstantially larger in cross section than the pool recirculatingconduit.
 3. The system of claim 1 wherein at least the side of saidcation selective membrane facing the anode is protected from attack bythe products of the anodic reaction by a porous, substantially inertdiaphragm in contact therewith.
 4. The system of claim 1 wherein saidcooling means comprise a confined space located in the wall of said celland in association with the back side of an electrode, said space beingin communication with means for circulating a cooling fluidtherethrough.
 5. The system of claim 4 wherein the cooling fluid iswater obtained through conduit means communicating with the said poolwater recirculation conduit.
 6. The system of claim 1 wherein the saidgas-liquid conduit tubes measure internally about three-sixteenth inchin diameter and between 9 to 12 inches in length.